📚 Ultrasound Scanning | 超声波扫描
Ultrasound scanning is a non-invasive medical imaging technique that uses high-frequency sound waves, typically in the range of 1 to 15 MHz, to visualise internal organs and tissues. It has become a cornerstone of diagnostic medicine because it avoids ionising radiation and provides real-time images of soft tissue structures, blood flow, and fetal development.
超声波扫描是一种非侵入性的医学成像技术，利用频率通常在 1 至 15 MHz 的高频声波来显示内部器官和组织。由于它避免了电离辐射，并能实时显示软组织结构、血流和胎儿发育情况，已成为诊断医学的基石。
1. The Nature of Ultrasound | 超声波的特性
Ultrasound refers to sound waves with frequencies above the upper audible limit of human hearing, approximately 20 kHz. In medical diagnostics, the frequencies used are much higher – between 1 MHz and 15 MHz – to achieve the resolution needed to distinguish fine anatomical details. These waves are longitudinal mechanical waves; they travel by compressions and rarefactions of the medium, requiring particles to oscillate parallel to the direction of wave propagation.
超声波指的是频率超过人类听觉上限（约 20 kHz）的声波。在医学诊断中，使用的频率要高得多，通常在 1 MHz 到 15 MHz 之间，以获得分辨细微解剖结构所需的分辨率。这些波是纵波，通过介质的压缩和稀疏传播，粒子沿波的传播方向平行振荡。
Like all waves, ultrasound follows the wave equation v = fλ, where v is the speed of sound in the medium, f is the frequency and λ is the wavelength. The speed of sound in soft tissue is approximately 1540 m s⁻¹, giving wavelengths between about 0.1 mm and 1.5 mm across the diagnostic range.
和所有波一样，超声波遵循波动方程 v = fλ，其中 v 是介质中的声速，f 是频率，λ 是波长。在软组织中声速约为 1540 m s⁻¹，因此在诊断频率范围内，波长约在 0.1 mm 到 1.5 mm 之间。
2. Piezoelectric Transducers | 压电换能器
Ultrasound waves are generated and detected using a transducer based on the piezoelectric effect. Certain crystals, such as lead zirconate titanate (PZT), change shape when an electric field is applied across them. When an alternating potential difference is applied at the desired ultrasonic frequency, the crystal vibrates and produces a corresponding ultrasonic wave.
超声波的产生和接收是借助基于压电效应的换能器实现的。某些晶体，如钛酸铅锆（PZT），在施加电场时会改变形状。当以所需的超声频率施加交变电压时，晶体会振动并产生相应频率的超声波。
Conversely, when a returning echo strikes the crystal, it induces a mechanical deformation that generates an alternating voltage across the electrodes. This same transducer therefore acts as both a source and a detector in pulse-echo systems. The crystal is backed by a damping layer to shorten the pulse length and improve axial resolution.
反之，当返回的回波撞击晶体时，会引起机械形变，从而在电极两端产生交变电压。因此，同一个换能器在脉冲回波系统中既充当发射源，又充当探测器。晶体背面装有阻尼层，以缩短脉冲长度并提高轴向分辨率。
3. Acoustic Impedance | 声阻抗
Acoustic impedance, Z, is a property of a medium that determines how much of an incident ultrasound wave is reflected at a boundary. It is defined as the product of the density of the medium, ρ, and the speed of sound in that medium, c:
声阻抗 Z 是介质的一种属性，它决定了入射超声波在边界处被反射的多少。它被定义为介质密度 ρ 与该介质中声速 c 的乘积：
Z = ρ c
l for muscle, and 1.58 × 10⁶ rayl for blood. Air has an extremely low acoustic impedance (~460 rayl), which leads to almost total reflection at an air–tissue interface.
其国际单位是瑞利，1 瑞利 = 1 kg m⁻² s⁻¹。医学成像中的典型数值为：水 1.38 × 10⁶ rayl，肌肉 1.70 × 10⁶ rayl，血液 1.58 × 10⁶ rayl。空气的声阻抗极低（约 460 rayl），导致在空气–组织界面上几乎发生全反射。
4. Intensity Reflection Coefficient | 强度反射系数
When an ultrasound beam encounters a boundary between two media of different acoustic impedances, part of the wave is reflected and part is transmitted. The intensity reflection coefficient, R, gives the fraction of incident intensity that is reflected. For normal incidence:
当超声波束遇到声阻抗不同的两种介质之间的界面时，一部分波被反射，另一部分透射。强度反射系数 R 给出了被反射的入射强度比例。对于垂直入射：
R = ((Z₂ − Z₁) / (Z₂ + Z₁))²
A large difference in impedance produces a strong reflection, which is essential for imaging boundaries between tissues. For example, at a muscle–blood interface, the difference is small so only about 1% of the intensity is reflected, whereas at a tissue–air interface, almost 100% is reflected. This is why a coupling gel is used to eliminate air between the transducer and the skin.
阻抗差异大会产生强反射，这对于对组织间的边界成像至关重要。例如，在肌肉–血液界面上，阻抗差异很小，只有约 1% 的强度被反射；而在组织–空气界面上，几乎 100% 被反射。这就是为什么要在换能器与皮肤之间使用耦合凝胶来排除空气。
5. Pulse-Echo Principle and A-scan | 脉冲回波原理与 A 型扫描
The fundamental operating mode of medical ultrasound is the pulse-echo technique. The transducer emits short bursts of ultrasound, typically lasting a few microseconds, and then switches to reception mode to detect echoes returning from tissue interfaces.
医学超声波的基本工作模式是脉冲回波技术。换能器发射持续时间通常为几微秒的短超声波脉冲，然后切换到接收模式，以检测从组织界面返回的回波。
The time delay between transmission and reception is measured. Since the speed of sound in tissue is known, the depth of a reflecting boundary can be calculated using d = v × t / 2, where t is the round-trip time. The factor of 2 arises because the pulse travels to the reflector and back.
测量发射与接收之间的时间延迟。由于已知组织中的声速，可以利用 d = v × t / 2 计算出反射界面的深度，其中 t 是往返时间。因子 2 的出现是因为脉冲先到达反射体，再返回换能器。
In an A-scan, the received echo signals are displayed as a graph of echo amplitude against time, which corresponds to depth. Such a display allows precise measurement of distances, for example, the cornea-to-lens distance in ophthalmology.
在 A 型扫描中，接收到的回波信号显示为回声幅度随时间（相当于深度）变化的曲线图。这种显示方式可以精确测量距离，例如在眼科中测量角膜到晶状体的距离。
6. B-scan and 2D Imaging | B 型扫描与二维成像
A B-scan builds a two-dimensional cross-sectional image by combining multiple A-scan lines. In a B-scan, the brightness of a spot on the display is proportional to the echo intensity, rather than being plotted as a separate trace. As the ultrasound beam is moved or swept across the body, a series of lines is acquired and used to form a 2D image of the anatomy.
B 型扫描通过将多个 A 型扫描线组合起来，形成二维横截面图像。在 B 型扫描中，显示屏上亮点的亮度与回波强度成比例，而不是绘制为独立的曲线。当超声波束在身体上移动或扫过时，会获取一系列线条，用于构建解剖结构的二维图像。
Modern real-time B-scanners employ arrays of multiple crystal elements (linear, curvilinear or phased arrays) that can electronically steer and focus the beam. By firing the elements with minute time delays, the beam can be swept rapidly to produce moving images of the heart, foetus or other moving structures.
现代实时 B 型扫描仪采用了由多个晶体振元组成的阵列（线阵、凸阵或相控阵），可以通过电子方式控制波束的偏转和聚焦。通过施加微小的时间延迟激励各振元，便可快速扫查波束，生成心脏、胎儿或其他运动结构的动态图像。
7. Doppler Ultrasound | 多普勒超声
Doppler ultrasound exploits the frequency shift that occurs when ultrasound is scattered by moving red blood cells. If blood is moving towards the transducer, the reflected frequency is higher than the transmitted frequency; if moving away, the reflected frequency is lower.
多普勒超声利用的是超声波被运动的红细胞散射时产生的频率偏移。如果血液朝向换能器运动，反射频率会高于发射频率；如果背向换能器运动，反射频率则低于发射频率。
The frequency shift, Δf, is given by the Doppler equation for reflected ultrasound:
频率偏移 Δf 由反射超声的多普勒方程给出：
Δf = 2 f₀ v cos θ / c
where f₀ is the transmitted frequency, v is the speed of the blood, c is the speed of sound in tissue, and θ is the angle between the beam and the direction of flow. The factor of 2 accounts for the frequency shift occurring both when the blood receives the ultrasound and when it re-emits it.
其中 f₀ 为发射频率，v 为血流速度，c 为组织中的声速，θ 为波束与血流方向之间的夹角。系数 2 是因为频率偏移既发生在血液接收超声波时，也发生在血液重新发射超声时。
Doppler ultrasound is used extensively to assess blood flow in arteries and veins, detect blockages, and monitor foetal heart rate. Colour Doppler overlays flow information onto a B-scan image, coding direction and speed by colour.
多普勒超声广泛用于评估动脉和静脉中的血流、检测堵塞以及监测胎儿心率。彩色多普勒将血流信息叠加到 B 型图像上，用颜色编码方向和速度。
8. Attenuation of Ultrasound | 超声的衰减
As ultrasound travels through tissue, its intensity decreases exponentially due to absorption and scattering. This attenuation is described by the relation:
超声波在组织中传播时，其强度因吸收和散射而呈指数衰减。这种衰减用下式描述：
I = I₀ e−μx
where I₀ is the initial intensity, μ is the linear attenuation coefficient, and x is the distance travelled. The attenuation coefficient depends on the tissue type and increases with frequency. High frequencies give better resolution but are attenuated more rapidly, limiting the depth of penetration.
其中 I₀ 为初始强度，μ 为线性衰减系数，x 为传播距离。衰减系数取决于组织类型，并且随频率升高而增大。高频提供更好的分辨率，但衰减更快，从而限制了探测的深度。
A convenient parameter is the half-value thickness, x½, the distance over which the intensity falls to half its original value, given by x½ = ln 2 / μ. Typical values in soft tissue are about 0.6 cm at 5 MHz, decreasing to a few millimetres at 15 MHz.
一个便利的参数是半值厚度 x½，即使强度降到初始值一半所需的距离，由 x½ = ln 2 / μ 给出。在软组织中，5 MHz 时的典型值约为 0.6 cm，在 15 MHz 时则减少到几毫米。
9. Resolution: Axial and Lateral | 分辨率：轴向与横向
Axial resolution is the ability to distinguish two reflectors positioned along the axis of the beam. It is determined by the spatial pulse length, which is the number of cycles in a pulse multiplied by the wavelength. A short pulse, achieved by heavy damping and high frequency, improves axial resolution. In practice, axial resolution is approximately half the spatial pulse length.
轴向分辨率是指区分沿波束轴向的两个反射体的能力。它由空间脉冲长度决定，即脉冲中的周期数乘以波长。通过强阻尼和高频获得的短脉冲可提高轴向分辨率。实际中，轴向分辨率近似为空间脉冲长度的一半。
Lateral resolution is the ability to distinguish two reflectors side by side, perpendicular to the beam axis. It is governed by the width of the ultrasound beam. Focusing the beam – either with a curved crystal or by electronic phased-array techniques – narrows the beam at the focal zone, thereby improving lateral resolution.
横向分辨率是指区分垂直于波束轴的两个并列反射体的能力。它受超声波束宽度的控制。通过聚焦（利用曲面晶体或电子相控阵技术）可缩窄焦点区域的波束，从而提高横向分辨率。
10. Safety and Intensity Considerations | 安全与强度考量
Diagnostic ultrasound is generally considered safe because it does not involve ionising radiation. However, the mechanical and thermal effects of ultrasound must be managed carefully. Two safety indices are displayed on modern scanners: the thermal index (TI), which estimates the potential temperature rise in tissue, and the mechanical index (MI), which estimates the likelihood of cavitation induced by the alternating pressure of the wave.
诊断超声通常被认为是安全的，因为它不涉及电离辐射。然而，超声的机械效应和热效应必须谨慎管理。现代扫描仪上会显示两个安全指数：热指数 (TI)，用于估算组织内可能的温升；机械指数 (MI)，用于估算波的交变压力引起空化的可能性。
Intensity is the power per unit area, averaged over different time intervals. Spatial-peak temporal-average intensity (ISPTA) is commonly used for safety guidelines. Operators are trained to use the minimum exposure time and the lowest power setting consistent with obtaining adequate diagnostic information – a principle known as ALARA (As Low As Reasonably Achievable).
强度是单位面积的功率，可在不同时间间隔内取平均值。空间峰值时间平均强度 (ISPTA) 常用于安全指南。操作人员接受培训，在获得足够诊断信息的前提下使用最短的照射时间和最低的功率设置——此原则称为 ALARA（在合理可行的范围内尽量低）。
11. Advantages and Limitations | 优势与局限
Ultrasound offers many advantages: it is relatively inexpensive, portable, and provides real-time imaging without the risks of ionising radiation. It is particularly suited to soft tissue imaging and is the modality of choice for obstetrics and cardiology. Doppler modes provide functional information about blood flow.
超声具有许多优势：成本相对较低、便携，并能实时成像，且没有电离辐射的风险。它特别适合软组织成像，是产科和心脏科的首选成像方式。多普勒模式还能提供有关血流的功能信息。
However, it also has limitations. Ultrasound cannot penetrate bone or air-filled spaces effectively, limiting its use in the adult brain or lungs. Image quality is highly operator-dependent, and resolution and penetration depth are inversely related. Obese patients can be challenging due to greater attenuation of the beam.
然而，它也存在局限。超声波无法有效穿透骨骼或充满空气的空腔，这限制了其在成人脑部或肺部的应用。图像质量高度依赖操作人员，且分辨率与探测深度呈反比关系。肥胖患者由于波束衰减更强，成像也更具挑战。
12. Summary | 总结
Ultrasound scanning combines the physics of wave propagation, piezoelectricity and signal processing to produce clinically invaluable images. Key concepts include acoustic impedance matching, reflection coefficients, the pulse-echo principle, A-scan, B-scan and Doppler shift. The inevitable trade-off between resolution and penetration is managed by selecting an appropriate transducer frequency. With strict controls on intensity and exposure time, diagnostic ultrasound remains a safe, versatile and indispensable tool in modern medicine.
超声波扫描将波的传播、压电效应和信号处理等物理学知识结合起来，产生具有重要临床价值的图像。核心概念包括声阻抗匹配、反射系数、脉冲回波原理、A 型扫描、B 型扫描以及多普勒频移。通过选择合适的换能器频率，可以平衡分辨率与穿透深度之间不可避免的取舍。在严格控制强度和照射时间的前提下，诊断超声仍然是现代医学中安全、多功能且不可或缺的工具。
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